Chemistry Reference
In-Depth Information
E5.4: Reaction of Alkaline Metals with Oxygen
Problem: The tarnishing of alkaline metals (see E5.3) is being interpreted by
students as a reaction with air or oxygen, respectively. Combustion in air is being
assumed and this assumption is to be tested by igniting a piece of the metal.
Material: Tripod with wire gauze, burner, little pieces of three alkaline metals (C/F).
Procedure: Under the hood one piece of metal at a time is to be put on the wire
gauze and ignited with the burner's roaring flame.
Observation: Lithium burns with a red, sodium with a yellow and potassium with a
violet flame; white smoke ascends, which can be pungent in the nose. White metal
oxides remain as combustion products (in case of contamination with kerosene, the
product might also appear black).
E5.5: Reaction of Alkaline Metals with Water
Problem: As discussed in E5.4, students might assume that water vapor is the reason
for the reaction of the metal cut surface in air. A reaction of alkaline metals with
water can be formulated as a hypothesis and tested in experiments.
Material: 3 beakers, 3 watch glasses, universal indicator paper, phenolphthalein
indicator solution, big glass dish; little pieces of the three alkaline metals (C/F).
Procedure: Three beakers are half-full with water. One piece of metal is to be put in
every beaker, the beaker is to be covered with a watch glass (caution: caustic
substance might spurt!). The solutions are to be tested with universal indicator
paper after the reactions. A part of the solution is to be concentrated by evaporation,
and the resulting white solids are to be tested with indicator paper.
In a second experiment a big glass dish is to be filled with water (about 2 cm high),
the glass dish is to be put on the switched on overhead projector. Two or three
pieces of sodium are to be put on the water surface; phenolphthalein indicator
solution may be added.
Observation: All three metals react with water, the intensity of the reaction
increases from lithium over sodium to potassium. During the reaction the piece of
sodium melts to a ball, potassium melts and ignites, the burning potassium delivers
caustic white smoke.
Three pieces of sodium circulate fizzy on the water surface and leave streaks (well
shown in the overhead projection). The streaks are colored dark red in case of the
added indicator solution.
All three of the solutions react highly alkaline. White solids remain after the water
evaporates. These also react alkaline with wet indicator paper.
Please note: Covering the beaker with a watch-glass serves as safety measure:
sodium or potassium balls might stick to the rim of the glass, explode and shoot out
of the glass.
Pieces of sodium can also be put on wet filter paper (under the hood!): in this case
they ignite with the formation of white smoke and react directly to white solid
hydroxide. Caution: the liquid metal may splash around - wear safety goggles!
E5.6: Detection of Hydrogen in the Lithium-Water Reaction
Problem: The sizzling sound and the burning potassium flame show the burning gas
that generates on the water surface during the metal-water reaction. The gas can be
